In enterprise SAN (Storage Area Network) environments, sequential I/O (input/output) is frequently implemented by certain applications. Sequential I/O is typical of large file reads and writes, and typically involves operating on one block immediately after its neighbour. With this type of I/O, there is little penalty associated with the disk drive head having to move to a new location.
An application (for example) running on the data processing system may wish to read from a file stored on the storage device. Typically, data is stored on the storage device in blocks, and the file may reside in one or more blocks. When a data query is made by the application in a data processing system to the storage device, the appropriate blocks are retrieved from the physical disks and stored in the cache of the storage device. From here, the blocks are passed on to the applications as IO buffers (or packets) via the physical media (for example, Fibre channel physical media).
In a storage area network, there may be multiple paths through the switched fabric from the data processing system to the storage device.
The storage device may have number of logical units (LUNs). A LUN, or Logical Unit Number (also referred to as Logical Unit i.e., LU), corresponds to an actual or virtual portion of a storage element. For example, a LUN may correspond to one or more disks in a disk drive array.
The storage device may include a cache to store recently accessed data. The storage device may also employ data prefetching. With data prefetching, the storage device fetches more data from the LUN than requested and stores them in the cache. When sequential requests (specifically reads) are detected, data blocks that are further along the sequence but have not yet been requested are copied from LUN into the cache. When the storage device receives requests for these prefetched data blocks, these blocks are provided more quickly to the requesting data processing system as access to cache is typically faster than access to physical storage.
The caching policies employed by different storage arrays are not the same. For example, some arrays employ caching for I/O passing through a port, caching for I/O passing through a controller or caching for I/O to/from any specific component (e.g. a LUN) in an array.
Under the above conditions, routing the 10 to components sharing the same cache may lead to cache invalidation, thus not effectively using data pre-fetching feature.
It is an object of embodiments of the invention to at least mitigate one or more of the problems of the prior art.